Improve precedence test

The jobs were starting in the correct order, but because they all
ran in parallel, they had a race to finish, and the build history
is built from finished jobs, which is what the test checked.

Instead, only allow one job at a time to run.

Change-Id: Ie09369d7c27b286c2339c9cc40658f8d5f629488
Merge "Improve precedence test"